Method of manufacturing yoke of rotary electric machine

ABSTRACT

A method of manufacturing a yoke includes a step of cutting a steel plate into a rectangular sheet having a plurality of first dovetail convexities at one end thereof and a plurality of second dovetail convexities at the other end, a step of rolling the rectangular sheet into a cylinder to fit the first dove tails and the second dove tails to each other and a step of punching border portions of the first dove tails and the second dove tails. The second dovetail convexities are formed to be the same in shape as the first dovetail convexities so that they can fit to the first dovetail convexities. A smooth-faced yoke can be manufacture without using an expensive welder.

CROSS REFERENCE TO RELATED APPLICATION

The present application is based on and claims priority from thefollowing Japanese Patent Applications: 2000-277074, filed Sep. 12,2000; 2000-323201, filed Oct. 23, 2000; and 2000-394893, filed Dec. 26,2000; the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of manufacturing a stator yokeof a rotary electric machine.

2. Description of the Related Art

Japanese Patent application JP-A-64-60247 discloses a method ofmanufacturing a yoke. In the method, a sheet made of magnetic material,such as iron or steel, is rolled and a number of convexities formed atone side thereof are fitted to the same number of concavities.Thereafter, respective central portions of the convexities of one sideof the steel sheet are punched and expanded, thereby clamping theopposite sides of the steel sheet. It is also known that the convexitiesare shaped into dovetail convexities.

However, because the convexities on a side of a steel sheet are expandedand deformed while the concavities on the other side are not expanded,differences in shape and thickness arise between the one side and theother side. This may cause the circularity of the yoke to be inaccurate.If the circularity is not accurate, it is not possible for a rotaryelectric machine to provide a small air gap between the rotor and thestator thereof. If the air gap is not made small, the performance of arotary electric machine lowers.

As a conventional technology of manufacturing a stator yoke of a rotaryelectric machine, there is a method of manufacturing a yoke disclosed inJP-A-5-91700. In this method, a sheet member is rolled to meet theopposite sides of the sheet member together, and the opposite sides arewelded together. For this purpose, positioning concavities are formed onthe surface of the sheet member. However, an expensive laser beam has tobe used to weld the opposite sides.

It is also known that dovetail convexities are formed at one side of thesheet member, which are fitted and clamped to concavities formed at theother side of the sheet member. In this case, sealant is filled in theclamped portions to ensure the seal, as disclosed in JP-A-52-20207.However, extra steps of filling sealant and removing leftover sealantare necessary, and work time and production cost increase.

Further, a yoke of a motor, such as a starter motor, is manufacturedfrom a steel plate, which is rolled up to form a cylindrical shape sothat the opposite sides of the plate are put together and welded to eachother as disclosed in JP-U-58-97957 and JP-A-64-60247. There is achamfered corner at the inside of the yoke in order to have a bracket orthe like fitted in the open end of the yoke smoothly.

Such chamfered corner of the yoke is formed by a machine after the steelplate is rolled up. Therefore, the manufacturing cost is high.

SUMMARY OF THE INVENTION

A main object of the present invention is to provide a method ofmanufacturing a yoke having an accurate circularity.

A main feature of the invention is a step of clamping in which theborder portions of both dovetail convexities are punched.

It is only necessary for the first dovetail convexities and the seconddovetail convexities to have a wider tail end than a tail base so thatit can fit to each other. It is not necessary for them to have atrapezoidal shape. It is not necessary that the step of cutting a steelsheet, the step of fitting the dovetail convexities and the step ofclamping the dovetail convexities are separately carried out. Further,it is not necessary that machines for respective steps are separate fromeach other. It is also possible to integrate such machines so that theycan carry out a series of the steps successively.

Because the punches are given along the border of both the first andsecond dovetail convexities to expand uniformly in the clamping step,both sides of the steel sheet are deformed to the same degree.Therefore, a simple and inexpensive method that is comprised of the stepof rolling a steel sheet, the step of meeting the opposite sides thereoftogether and the step of clamping them can improve the circularity ofthe yoke. Further, a cylindrical yoke having an accurate circularity canbe manufactured from a steel sheet at a comparatively low cost. Inaddition, the first and second dovetail convexities are respectivelyformed at almost the whole length of the opposite sides thereof andclamped. Therefore, the fastening strength is increased.

According to another feature of the invention, both the first and seconddovetail convexities are the same in size and shape. They are givenpunches on the border equally, and they are deformed equally so that avery accurate circularity of the yoke can be provided.

According to another feature of the invention, the first and seconddovetail convexities are given punches only on the middle borderportions where a straight line crosses, and the clamping can becompleted by a smallest number of punches. In addition, work time of theclamping can be reduced because the positions on which the punches aregiven are on a straight line.

Therefore, the manufacturing cost and work time can be reduced.

According to another feature of the invention, punches are given on themiddle of the longitudinal borderline to clamp the opposite sides of therolled steel sheet. Therefore, compressed stress or bending strain doesnot appear at the axial ends of the borderline, and the opposite sidesof the rolled steel sheet are well prevented from separating from eachother at the axial ends of the yoke.

According to another feature of the invention, the profile of the firstand second dovetail convexities includes a straight portion. Therefore,the steel sheet can be cut easily at a low cost. In other words, sincethe dies of the press machine includes the corresponding straightportion, the dies can be made at a low cost. Therefore, work time andthe cost of production can be reduced.

Another object of the present invention is to provide a simple method ofmanufacturing a well-sealed yoke at a low cost.

According to a feature of the invention for this object, an outerperiphery of a yoke is electro-statically painted in a painting step toseal portions where convexities and concavities are fitted to eachother. Since the outer periphery of the yoke is electro-staticallypainted, small gaps can be sealed by paint. Therefore, a sufficienthermetic seal can be provided. Since the surface of the clamped portionsealed by the electro-static painting is smooth, no step of smoothingthe yoke surface is necessary.

According to another feature of the invention, a gap opening to theouter periphery is formed at least the clamped portions. Therefore,electric charge gathers around the gap when the peripheral surface ofthe yoke is electro-statically painted, and the paint moving to theclamped portion gets into the gap.

Therefore, a very smooth peripheral surface of the yoke can be provided.

According to another feature of the invention, the gap is less thantwice as thick as a paint film formed in the painting step, the gap canbe sufficiently filled with the paint.

Accordingly, the yield rate of the paint can be improved and theproduction cost and time can be reduced.

According to another feature of the invention, the yoke is heated beforepowder paint is sprayed on the peripheral surface, the paint can stickto the peripheral surface very well. Therefore, a beautiful yoke can beprovided.

According to another feature of the invention, the starting point of thepainting and the ending point of the painting are set at the clampedportion so that the clamped portion can be painted twice. Therefore, thegaps at the clamped portion can be sufficiently filled with paint, and ahigher hermetic yoke can be provided without additional cost.

Another object of the invention is to provide a yoke of a rotaryelectric machine that can eliminate the machine work for the chamfering.

A yoke of a motor is formed from a steel plate that is rolled to form acylindrical shape having axially opposite ends that are chamfered atinside or outside corners thereof. According to a feature of theinvention, the steel plate is chamfered before it is rolled up.

This invention omits additional chamfering machine work. If a member(e.g. a bracket) to be assembled to the yoke is fitted to the inside ofthe yoke in the axial direction thereof, the inside corner of the yokeis chamfered. On the other hand, the outside corner of the yoke ischamfered if the member is fitted to the outside of the yoke, so thatcracking from the shear drop can be prevented.

According to another feature of the invention, the steel plate is cut bya press machine into a strip having a fixed width and rolled so as tolocate a shear drop that is formed due to cutting by a press machine atthe outside of said cylindrical shape. Accordingly, the cut surface doesnot expand circumferentially outer direction when the steel plate isrolled up so that cracking can be prevented from extending from the cutsurface.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and characteristics of the present invention aswell as the functions of related parts of the present invention willbecome clear from a study of the following detailed description, theappended claims and the drawings. In the drawings:

FIG. 1 is a plan view of a cut sheet for a yoke according to a firstembodiment of the invention;

FIG. 2 is a fragmentary enlarged view of first dovetail convexities ofthe steel sheet shown in FIG. 1;

FIG. 3 is a perspective view of a yoke after a fitting step according tothe first embodiment;

FIG. 4 is an enlarged side view of the dovetail convexities of the yokeshown in FIG.3 being fitted together;

FIG. 5 is a side view of punched portions of the dovetail convexitiesshown in FIG. 3 in a clamping step;

FIG. 6 is a cross-sectional view of a tool used in the clamping stepaccording to the first embodiment;

FIG. 7 is a side view of portions around the dovetail convexities forshowing a plastic deformation in the clamping step according to thefirst embodiment;

FIG. 8 is a perspective view of the yoke after the clamping stepaccording to the first embodiment.

FIG. 9 is a side view of punched portions of the dovetail convexities ina clamping step according to a variation of the first embodiment;.

FIG. 10 is a side view of punched portions of the dovetail convexitiesin a clamping step according to another variation of the firstembodiment;

FIG. 11 is a side view of punched portions of the dovetail convexitiesin a clamping step according to another variation of the firstembodiment;

FIG. 12 is a schematic side view typically illustrating a drawback inthe clamping step.

FIG. 13 is a side view of punched portions of the yoke around thedovetail convexities in a clamping step according to a second embodimentof the invention;

FIG. 14 is an enlarged side view of the yoke by a method according to athird embodiment;

FIG. 15 is a plan view illustrating a cut steel sheet in a cutting stepof a method according to a fourth embodiment of the invention;

FIG. 16 is a perspective view illustrating a hollow cylindrical yokeaccording to the fourth embodiment;

FIG. 17 is a fragmentary cross-sectional view illustrating a portionaround a gap of the yoke after a tube-forming step according to thefourth embodiment;

FIG. 18 is a cross-sectional schematic diagram illustrating a holderholding the yoke in a painting step according to the fourth embodiment;

FIG. 19 is a perspective schematic diagram illustrating the paintingstep according to the fourth embodiment;

FIG. 20 is a fragmentary cross-sectional view of the yoke manufacturedby the method according to the fourth embodiment;

FIG. 21 is a fragmentary cross-sectional view illustrating a portion ofthe yoke with paint powder sticking thereto;

FIG. 22 is a schematic cross-sectional diagram illustrating a holderholding the yoke in the painting step according to a variation 1 of thefourth embodiment;

FIG. 23A is a plan view of a cut steel sheet for forming a yoke, andFIG. 23B is a perspective view illustrating a yoke manufactured by amethod according to a variation 2 of the fourth embodiment;

FIG. 24A is a side view of a cut steel plate for a fifth embodiment ofthe invention, and FIG. 24B is a plan view thereof;

FIG. 25 is a cross-sectional view of a yoke and a bracket fittedtogether;

FIG. 26 A is a perspective view illustrating the whole portion of theyoke shown in FIG. 25, and FIG. 26B is a cross-sectional viewillustrating an end of the yoke;

FIG. 27 is a cross-sectional view illustrating the yoke and the bracketfitted together;

FIGS. 28A and 28B are side and front views illustrating the cut surfaceof the steel plate cut by a press machine; and

FIG. 29A is a cross-sectional view of an end of the yoke, and FIG. 29Bis a plan view thereof viewed in the axial direction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Methods of manufacturing a yoke according to various embodiments of theinvention are described with reference to the appended drawings.

A method of manufacturing a stator yoke of a starter motor according toa first embodiment of the invention is described hereafter. The methodof manufacturing a yoke includes steps of cutting, fitting and clamping.

At the cutting step, a belt like long steel sheet is cut into arectangular shape having first dovetail convexities 1 on one sidethereof and second dovetail convexities 2 on the other side so that theycan be fitted to the first dovetail convexities. A press cutter is usedto cut the steel sheet in the following manner to have a good yieldrate: when the first dovetail convexities 1 are cut at one side, thesecond dovetail convexities 2 are automatically formed at the otherside, as the fragments of the first dovetail convexities.

As shown in FIG. 1, there are three first dovetail convexities 1 formedat the middle and half-dovetail fragments 1′ and straight portions atthe opposite axial ends, on one side (right in FIG. 1) of the cut steelsheet. There are four concavities 10 among the three dovetailconvexities 1 and two fragments 1′. On the other hand, there are foursecond dovetail convexities 2 at the middle thereof and incompleteconcavities 20′ at the opposite axial ends on the other side (left inFIG. 1). There are three concavities 20 among the second dovetailconvexities 2.

Three first dovetail convexities 1 on one side and four second dovetailconvexities 2 on the other side are the same in shape and formed ataxial positions so that they can fit each other. In other words, thefirst dovetail convexities 1 and incomplete dovetail fragments 1′ on oneside are located to correspond to the concavities 20 and the incompleteconcavities 20′ and to fit to the concavities 20 and the incompleteconcavities 20′. Similarly, the second dovetail convexities 2 on theother side are located to correspond to the concavities 10 and to fit tothe concavities 10.

Each of the first dovetail convexities 1, as shown in FIG. 2, has asemicircular neck 11 at the base thereof and a pair of semicircular ears12 at the end thereof. The semicircular neck 11 has a radius R′ that isapproximately the same as or a little smaller than a radius R of theears 12. The second dovetail convexities 2 are the same in shape as thefirst dove tails 1 except for projecting direction and the axialposition.

Each of the first dovetail convexities 1 has an axially extendingstraight portion 13 at the end thereof. Straight portions 14 are alsoformed at concavities 10 between the first dovetail convexities 1 andthe half dovetail fragment 1′. Since the second dovetail convexities 2and the first dovetail convexities 1 are the same in shape, a portion ofthe first and second dovetail convexities is straight.

In the step of cutting, as shown in FIG. 3, the cut steel sheet isrolled to be cylindrical, one side of the steel sheet and the other sideare put together to fit the first dovetail convexities and the seconddovetail convexities to each other. As shown in FIG. 4, there is a smallgap g between each of the first dovetail convexities 1 and theneighboring one of the second dovetail convexities 2, so that the firstdovetail convexities 1 and the second dove tails 2 can loose-fit to eachother.

Finally, in the step of clamping, as shown in FIG. 5, the first dovetailconvexities 1 and the second dovetail convexities 2, which are fitted toeach other, are punched at borders thereof so that one side of therolled steel sheet or a yoke Y and the other side thereof can be clampedto each other. In more detail, the first dovetail convexities 1 and thesecond dovetail convexities 2 are punched at middle portions of theborders located at a center line CL, as shown by circles, so that theone side and the other side of the yoke Y can be clamped to each other.

As shown in FIG. 6, the opposite sides of the steel sheet for the yoke Yare set on a lower die U that has the same outside diameter as theinside diameter of the yoke Y. Then portions of the opposite sides arepunched by a cylindrical flat-head punch P from above, so that, as shownin FIG. 7, the first dovetail convexities 1 and the second dovetailconvexities 2 plastically deform or flow in the directions indicated byarrows to fill the gaps between the dovetail convexities 1 and 2. Thus,the dovetail convexities 1 and 2 are tightly clamped, and a cylindricalyoke Y is formed from a steel strip as shown in FIG. 8.

Thus the method of manufacturing a yoke according to the firstembodiment has the following effects.

When the first dovetail convexities 1 and the incomplete dovetailconvexities 1′ on one side of the steel sheet are cut out by a presscutter, the second dovetail convexities 2 and the incomplete concavities20′ are automatically formed on the other side. Therefore, the yieldrate of the steel sheet becomes higher and the material cost becomeslower; the straight portions 13 and 14 of the first and second dovetails 1 and 2 make the cost of the dies of the press cutter inexpensive.

As shown in FIG. 5, both the dovetail convexities 1 and 2 are the samein shape and are punched at the middle of the borders along the centerline CL of both the dovetail convexities 1 and 2. Therefore, both thedovetail convexities 1 and 2 plastically deform uniformly. As a result,a yoke Y can have an accurate circularity.

A small number (e.g. eight) of the punched positions (indicated bycircles) is sufficient for the clamping. Therefore, the cost and timefor the clamping step is small. Therefore, the method of manufacturing ayoke according to the first embodiment is suitable to mass-production ofthe yokes.

Because the first dovetail convexities 1 and the second dovetailconvexities 2 are clamped after being fitted each other, a greatfastening strength can be provided.

As a variation 1 of the present embodiment, the dovetail convexities 1and 2 are punched at the edges thereof located on the border line, asshown in FIG. 9. Therefore, the gap g between the first and seconddovetail convexities 1 and 2 can be filled and clamped although strengthof the punches is smaller. Therefore, a yoke Y that has a highercircularity than the first embodiment can be provided.

As a variation 2 of the invention, as shown in FIG. 10, punches aregiven at opposite sides of the edge along the borders of both thedovetail convexities 1 and 2. In this embodiment, the portions at whichpunches are given are disposed in a lattice shape. Therefore, theclamping device can be made easy. Although the punching force is smallerthan that of the first embodiment, the gap g between the first andsecond dovetail convexities 1 and 2 can be closed and clamped.Therefore, more accurate circularity of the yoke can be provided.

As a variation 3 of this embodiment, both the dovetail convexities 1 and2 are punched along the border to be clamped, as shown in FIG. 11.Although the punches are not so strong as the first embodiment, the gapg between the first and second dovetail convexities 1 and 2 can beclosed and clamped. Therefore, more accurate circularity of the yoke Ycan be provided.

In stead of a punch, a roller can be used in the step of clamping,

A method of manufacturing a yoke according to a second embodiment of theinvention is described hereafter.

If the dovetail convexities 1 and 2 formed near the axial edges arepunched, the incomplete dovetail convexities 1′ may deform in the axialdirection, and a gap G is formed between one and the other sides of thesteel sheet at the axial ends of the yoke Y as shown in FIG. 12.

In the method according to the second embodiment, the portions ataxially opposite ends of the first and second dovetail convexities 1 and2 are not punched, as shown in FIG. 13. That is, the portion along theborder of the incomplete dovetail 1′ and the second dovetail 2 is notpunched in the clamping step. Others are the same as the embodiment 1.

Therefore, the incomplete dovetail convexities 1′ fitted to the dovetailconvexities 2 at the axial ends of the yoke Y without deforming, so thatthe clamped portion is not separated. Further, the number of portionsbeing punched in the clamping step can be reduced from 8 to 6. Althougha small gap remains between the incomplete dovetail 1′ and the seconddove tail 2 at the axially opposite ends of the steel sheet, such gapdoes not affect magnetic characteristic of the yoke Y.

A third embodiment of the invention is described with reference to FIG.14. The fitting step is almost the same as the fitting step of the firstembodiment.

In the cutting step, inversed trapezoidal first dovetail convexities 1and the second dove tails 2 are respectively formed on one and the othersides of the steel sheet. Both the dovetail convexities 1 and 2 are thesame in shape, and each corner of the dovetail convexities 1 and 2 ischamfered.

In the clamping step, the corners of the dovetail convexities 1 and 2are punched and clamped. However, the corner of the incomplete dovetailconvexities 1′ and the corresponding corners of the dovetail 2 are notpunched. Therefore, the circularity of the yoke becomes more accurate,the joint strength can be improved.

Since the dovetail convexities 1 and 2 of this embodiment include morestraight profile portions than the first embodiment, the press cuttercan use less expensive dies.

The shape of the dovetail convexities 1 and 2 can be modified, in thecutting step, to many shapes other than the inversed trapezoid. Thepunching position can be changed in many ways.

A method of manufacturing a yoke according to a fourth embodiment isdescribed with reference to FIGS. 15-21.

The method of manufacturing a yoke according to this embodiment has atube-forming step and a pre-heating step and a painting step in thisorder, as described in detail below.

The tube-forming step includes a cutting step and a clamping step.

In the cutting step, a belt-like long plate is cut by a press machine ora press cutter into a rectangular sheet to form a plurality ofconvexities 101 at one side and a plurality of concavities 102 to befitted to the convexities 101 at the other side. When the convexities101 are formed at one side, the concavities 102 are automatically formedat the other end.

The one side of the cut sheet (upper side of FIG. 15) has threeconvexities 101 at the middle thereof, and the other side of the cutsheet (lower side of FIG. 15) has three concavities 102 at the middlethereof.

The three convexities 101 and the three concavities 102 arecomplementary in shape to each other and located so that they can fit toeach other.

The shape of the convexities 101 corresponds to a portion of an ellipsethat is cut by a line parallel to the major axis thereof. Each of theconvexities 101 has a neck 11 at the base thereof. The shape of theconcavities 102 is formed so that the convexities 101 can fit theconcavities 102 without gaps.

In the fitting step, the cut sheet is rolled up to be cylindrical, andthe one side and the other side of the sheet are put together to fit theconvexities 101 and the concavities 102 to each other, as shown in FIG.16.

In the clamping step, the jointed portions 103 of the convexities 101and the concavities 102 are punched to clamp the one and the other sidesof the rolled sheet to each other.

In the preheating step of this embodiment, the yoke Y is heated by aheating apparatus such as an air heating furnace, an infrared lamp, aninduction heating apparatus before it is painted.

In the painting step, the yoke Y formed in the tube-forming step is heldby a holder so that the outer periphery of the yoke iselectro-statically painted, as shown in FIG. 18.

The holder is comprised of a shaft 91, a pair of masking disks 92 thathas an outside diameter larger than the outside diameter of the yoke Y,a pair of support members 93 that has an approximately the same outsidediameter as the inside diameter of the cylindrical member Y and afastening member 94 for fixing the masking disks 92 and the yoke Y tothe shaft 91. The pair of masking disks 92 is electrically connected tothe yoke Y and is disposed to mask the opposite axial ends of the yokeY. The shaft 91 extends through the center of the masking disks 92 andis rotatably supported by a rotary device (not shown). The shaft 91 andthe pair of masking disks 92 are electrically connected to each otherwhen the yoke Y is held by the holder. Therefore, the yoke Y iselectrically connected to the shaft 91 while it is sandwiched by thepair of masking disks 92 at the opposite ends thereof.

The electro-static painting is carried out by an electro-static paintinggun. The electro-static painting gun includes a gun body 81 that has aninjection nozzle 811 for injecting electrically charged paint powder Pand an electric power unit 82 for supplying electric power to the gunbody 81. The injection nozzle 811 injects paint powder straight. Anegative electrode of the power unit 82 is connected to the gun body 81,and the positive electrode of the power unit 82 is grounded.

The electro-static painting is carried out while rotating the shaft 91of the holder to rotate the yoke Y after the holder for holding the yokeY is grounded. The injection nozzle 811 of the electro-static paintinggun is located to inject the paint powder P toward the cylindrical axisof the yoke Y. The electro-static painting is started from the portionjust in front of the jointed portion 103 around the whole outerperiphery of the yoke Y and ended at the same portion of the jointedportion 103, as shown in FIG. 5. The quantity of the paint is controlledso that the thickness of the paint film that is stuck on the outerperiphery of the yoke Y is less than twice as thick as the gap g.

Thus, the paint film L is formed on the yoke Y at the jointed portion103 so that the yoke Y has a hermetic seal, as shown in FIG. 20.

In the tube-forming step, when the convexities 101 are formed at a sideof a sheet by a press cutter, the concavities 102 are automaticallyformed. Therefore, the yield rate of the yoke material is high and thematerial cost can be reduced.

In the painting step, since electric charges gather at corners of thegap g of the yoke Y, the paint powder P concentrates at and sticks tothe portion around the gap so that the gap g can be fully closed by thepaint powder P, as shown in FIG. 21. In the painting step, since theyoke Y is preheated in the preheating step, the paint powder can wellstick to the outer periphery of the yoke, so that the exterior of theyoke can be improved.

As a variation 1 of the fourth embodiment, the holder used in thepainting step is formed from insulation material as a masking member 92′to electrically insulate the yoke Y from a shaft 91′, as shown in FIG.22. The electrical connection of the shaft 91′ and the yoke Y is made bya conductive support 93′c that is fixed and electrically connected tothe shaft 91′ and abutted against the inner periphery of the yoke Y.Since an electric potential is not applied to the masking member 92′,paint powder does not stick to the masking member 92′.

As a variation 2 of the fourth embodiment, the shape of each of theconvexities 101′ is an isosceles trapezoid, as shown in FIGS. 23A and23B. Since the convexities 101′ and the concavities 102′ are profiled bystraight lines, it is easy to cut those from a plate.

A method of manufacturing a yoke according to a fifth embodiment of theinvention is described with reference to FIGS. 24A and 24B-FIG. 27.

A yoke Y is used in a starter motor of a vehicle starter. As shown inFIG. 25, a bracket B is fitted to the inner periphery of the yoke Y.

The yoke Y is manufactured by rolling up the longitudinal sides (upperand lower sides) of the steel plate shown in FIGS. 24A and 24B to form acylindrical shape.

The steel plate is formed from a sheet material (not shown), which iscut into a rectangle having a fixed width by a press machine. As shownin FIG. 24A, a plurality of convexities (dovetail convexities) 201 isformed on one of the longitudinal sides, and a plurality of concavities202 is formed on the other side. The convexities 201 and the concavities202 are fitted to each other, as shown in FIG. 26A, after the steelplate is rolled up into a cylindrical yoke.

Chamfered corner C (for example, the slope of the corner is about 45degree) is formed at a corner of the steel plate in the width direction(right corner and/or left corner) before it is rolled up, as shown inFIG. 24B. The chamfered corner C is formed by a press machine at theinside corner of the yoke where the bracket B is fitted. In other words,the steel plate is rolled up so that the chamfered corner C can belocated at the inside of the yoke Y in the thickness direction thereof,as shown in FIG. 26B.

Therefore, it is not necessary to cut such a chamfered corner C by anadditional machine work, so that the manufacturing cost can be reduced.Especially, it is easy to form a chamfered corner by pressing before thesteel plate is rolled up. The chamfered corner can be formedconcurrently when the steel plate is cut by a press machine.

The bracket B can be fitted to the outside of the yoke Y, as shown inFIG. 27. In this case, the chamfered corner should be formed at theoutside corner of the yoke Y. The steel plate can be formed from a longbelt-like material having a fixed width by separating the portions ofthe convexities 201 and concavities 202.

A method of manufacturing a yoke according to a sixth embodiment of theinvention is described with reference to FIGS. 28A and 28B. The sheardrop 204 is formed on the outside of the yoke Y by cutting the steelplate by a press machine at opposite sides in the width direction. Inthis case, the shear drop 204 is formed at the upper side of the cutportion, and a cut surface 205 is formed next to the shear drop.

If the shear drop 204 is located inside the yoke Y, as shown in FIG.29A, the shear drop can be utilized as a chamfered corner of the yoke Yinto which the bracket B is fitted, as shown in FIG. 25. If the steelplate is rolled up so that the cut surface 205 can be located at theoutside of the yoke Y, the cut surface 205 is expanded in thecircumferential direction to help the cut surface 205 crack.

In this embodiment, the cut surface 205 is located at the inside of theyoke Y so that the chamfered corner C can be formed at the cut surface,as shown in FIG. 26B.

If the bracket B is fitted to the outside of the yoke Y, the chamferedcorner C should be formed at the outside of the yoke Y.

The chamfered slope is not limited to 45 degree. It may be 30, 60 or anyother degree. The chamfered slope may be round R.

In the foregoing description of the present invention, the invention hasbeen disclosed with reference to specific embodiments thereof. It will,however, be evident that various modifications and changes may be madeto the specific embodiments of the present invention without departingfrom the scope of the invention as set forth in the appended claims.Accordingly, the description of the present invention is to be regardedin an illustrative, rather than a restrictive, sense.

What is claimed is:
 1. A method of manufacturing a yoke comprising thesteps of: cutting a steel plate into a rectangular sheet to form aplurality of first dovetail convexities and concavities alternately atone end thereof and a plurality of second dovetail convexities andconcavities alternately to be loosely fitted to said plurality of firstdovetail convexities at the other end; rolling said rectangular sheetinto a cylinder to fit said plurality of first dovetail convexities andconcavities and said plurality of second dovetail convexities andconcavities to each other; and punching said plurality of first dovetailconvexities and said plurality of second dovetail convexities at middleportions of borders thereof generally equally at the same time so thatthe first and second convexities equally deform to fill gaps at theborders and tightly fit to each other.
 2. The method of manufacturing ayoke according to claim 1, wherein said first dovetail convexities andsaid second dovetail convexities are cut into the same shape in saidstep of cutting.
 3. The method of manufacturing a yoke according toclaim 1, wherein said first dovetail convexities and said seconddovetail convexities are punched only on a straight line crossing saidborder portions.
 4. The method of manufacturing a yoke according toclaim 1, wherein said first dovetail convexities a n d said seconddovetail convexities are punched except opposite ends of said borderportions.
 5. The method of manufacturing a yoke according to claim 1,wherein said first dovetail convexities and said second dovetailconvexities are shaped so that at least a portion of each dovetailconvexity is straight.